As nonvolatile semiconductor memory devices such as a NAND flash memory become finer, degradation in the data retention characteristic poses a problem.
To cope with this problem, there has been proposed a double floating gate structure in which a floating gate is formed from two layers. In the double floating gate structure, it is important to prevent electrons from moving from the upper floating gate electrode to the lower floating gate electrode.
In the conventional double floating gate structure, however, it is difficult to sufficiently prevent electron movement from the upper floating gate electrode to the lower floating gate electrode. It is therefore difficult to obtain a nonvolatile semiconductor memory device having an excellent data retention characteristic.